1. Field of the Invention
The present invention relates to a tripod type constant velocity universal joint used for transmitting a torque between rotation shafts that are incorporated into a drive system of an automobile and are mainly located on a nonlinear line, for example.
2. Description of the Related Art
In a drive shaft of an automobile, or the like, a constant velocity universal joint of a tripod type is frequently used as a constant velocity universal joint on an inboard side of the automobile. As shown in FIGS. 7 and 8, this tripod type constant velocity universal joint 31 includes an outer joint member 34, an inner joint member 37, and rollers 38. The outer joint member 34 is secured to an end of a first rotation shaft 32 such as a drive shaft, has a bottomed cylindrical shape, and includes track grooves 33 extending axially and distributed at circumferentially trisectional positions on its inner circumferential surface. The inner joint member 37 is secured to an end of a second rotation shaft 35 such as a driven shaft, and includes trunnion journals 36 extending radially and distributed at circumferentially trisectional positions on its outer circumferential surface. Each of the rollers 38 is supported by the associated trunnion journal 36 in such a manner that the roller 38 can rotate freely and be slightly displaced in the axial direction of the associated trunnion journal 36. Each roller 38 is received within the associated track groove 33 of the outer joint member 34 and can roll on roller guiding surfaces 33a of the track groove 33. The roller guiding surfaces 33a are formed on both circumferential sides of the track groove 33.
In the case where the central axis of the first rotation shaft 32 is not coincident with that of the second rotation shaft 35, i.e., the tripod type constant velocity universal joint 31 forms an operating angle, each trunnion journal 36 is displaced with respect to the roller guiding surface 33a of the corresponding track groove 33 in a direction in which that trunnion journal 36 swings around the inner joint member 37 along with the rotation of the first and second rotation shafts 32 and 35, as shown in FIGS. 7 and 8. In this displacement, the roller 38 supported by each trunnion journal 36 rolls on the roller guiding surface 33a of the corresponding track groove 33.
In this case, each roller 38 moves along the roller guiding surface 33a while changing its tilt with respect to the axial direction of the outer joint member 34, and is displaced in the axial direction of the corresponding trunnion journal 36. When each roller 38 makes such a complicated movement, the outer circumferential surface of each roller 38 is not smoothly moved with respect to the corresponding roller guiding surface 33a, thus causing large friction between those surfaces. This causes generation of the 3rd order cyclic axial force in the tripod type constant velocity universal joint 31. This axial force may cause vibration called as shudder, for example, in the case where a large torque is transmitted while the tripod type constant velocity universal joint 31 is incorporated into an automobile or the like and forms a large operating angle.
Means for solving the above problem is disclosed in U.S. Patent Application Publication No. US2002/0115491 A1 and Japanese Patent Laid-Open Publication No. Hei 5-215141, for example. Those publications describe a so-called double-roller tripod type constant velocity universal joint in which an outer roller, which can make a relative rotation with respect to an inner roller via a needle roller, rolls along an track groove of an outer joint member, as shown in FIGS. 9 and 10.
In the tripod constant velocity universal joint 41 disclosed in US2002/0115491 A1, an outer circumferential surface of a head of a trunnion journal 42 is formed to be spherical and an inner circumferential surface of the inner roller 43 is also formed to be spherical, so that both of those surfaces can spherically fit with each other (see FIG. 9). Thus, it is possible to ensure a sufficient contact area between the trunnion journal 42 and the inner roller 43 and reduce the pressure on the contact area. This leads to increase of rotation durability.
On the other hand, the tripod type constant velocity universal joint 51 disclosed in Japanese Patent Laid-Open Publication No. Hei 5-215141, PCD of the trunnion journal and that of the track groove of the outer joint member are set to have a plus offset (i.e., journal PCD>track-groove PCD) or a minus offset (i.e., journal PCD<track-groove PCD), thereby reducing and stabilizing an induction force at every operating angle. The roller guiding surface 56a is formed to have a Gothic arch shape and is in angular contact with an outer circumferential surface of the outer roller 54 at two points (see FIG. 10). This angular contact allows generation of a couple of restoring forces acting in a direction for preventing tilt of the outer roller 54 on a plane containing the central axis of the outer joint member 56 to place the outer roller 54 in parallel to that plane. Thus, it is possible to keep the outer roller 54 parallel to the track groove of the outer joint member 56.
However, when the aforementioned double-roller tripod type constant velocity universal joint is rotated with a certain operating angle, each trunnion journal swings on the track groove and is also displaced in the radial direction of the outer joint member. Thus, a tilting moment is generated so as to cause the outer roller to tilt in a cross section perpendicular to the central axis of the outer joint member. Simultaneously, a spinning moment is also generated by a frictional force between the inner circumferential surface of the inner roller and the outer circumferential surface of the trunnion journal caused by the swinging of the trunnion journal so as to cause the outer roller to be tilted in a cross section containing the central axis of the outer joint member. As those moments causing the tilt of the outer roller become larger, the outer roller largely tilts. Consequently, the roller guiding surface of the track groove, which is on the no-load applied side, comes into contact with the outer circumferential surface of the outer roller, or a guide shoulder surface formed at the bottom of the track groove comes into contact with the side surface of the outer roller. Moreover, when the outer roller tries to tilt further, the contact surface pressure on the area between the outer roller and the roller guiding surface of the track groove on the no-load applied side or the guide shoulder surface at the bottom of the track groove further increases.
In this case, the frictional force within the tripod type constant velocity universal joint is increased, thus increasing the 3rd order cyclic axial force. This may cause generation of shudder or degradation of rotation durability.